One-Center Charge Transfer Transitions in Manganites

الأصل · EN

In frames of a rather conventional cluster approach, which combines the crystal field and the ligand field models we have considered different charge transfer (CT) states and O 2p-Mn 3d CT transitions in MnO₆⁹⁻ octahedra. The many-electron dipole transition matrix elements were calculated using the Racah algebra for the cubic point group. Simple "local" approximation allowed to calculate the relative intensity for all dipole-allowed π-π and σ-σ CT transitions. We present a self-consistent description of the CT bands in insulating stoichiometric LaMn³⁺O₃ compound with the only Mn³⁺ valent state and idealized octahedral MnO₆⁹⁻ centers which allows to substantially correct the current interpretation of the optical spectra. Our analysis shows the multi-band structure of the CT optical response with the weak low-energy edge at 1.7 eV, associated with forbidden t₁g(π)-eg transition and a series of the weak and strong dipole-allowed high-energy transitions starting from 2.5 and 4.5 eV, respectively, and extending up to nearly 11 eV. The most intensive features are associated with two strong composite bands near 4.6÷4.7 eV and 8÷9 eV, respectively, resulting from the superposition of the dipole-allowed σ-σ and π-π CT transitions. These predictions are in good agreement with experimental spectra. The experimental data point to a strong overscreening of the crystal field parameter Dq in the CT states of MnO₆⁹⁻ centers.